Rolling stands frequently incorporate roll eccentricities, caused for example by inaccurately worked support rolls or by imprecise mounting of the support rolls, which adversely affect the quality of the rolled strip, the roll eccentricities being expressed in the strip with the rotational speed of the rolls affected by eccentricity, normally the support rolls, depending on the stiffness of the rolling stand and the rolling stock. The frequency spectrum of the eccentricities and of the disruptions in the strip caused by them essentially contains the fundamental frequencies of the top and bottom support rolls; but higher harmonic oscillations are also present, although they frequently only make an appearance at a reduced amplitude. Due to slightly different diameters and rotational speeds of the upper and lower support rolls, the frequencies assigned to the support rolls may diverge from each other.
EP 0 170 016 B1 discloses a method of the type referred to in the introduction, where the influence of roll eccentricities in the position or thickness regulation of rolling stands is compensated for, the roll eccentricities being identified on the basis of a measurement of the rolling force in the rolling stand. Oil pressure sensors are normally used for measuring the rolling force, the measured values from which sensors are distorted to a considerable degree by friction effects. This means that no adequately reliable and effective suppression of the influence of roll eccentricities can be effected with the aid of the measuring instruments. More reliable and more accurate measuring methods for the rolling force are too costly and too complex.
An approach known from EP 0 698 427 B1, in a method for suppressing the influence of roll eccentricities, is to use the run-out thickness of the rolling stock instead of the rolling force as a measured value. Thickness sensors are very costly, however, and therefore, in the case of multi-stand rolling mills, are normally only provided upstream and downstream of the first rolling stand and downstream of the last one.
A method for suppressing the influence of roll eccentricities on the run-out thickness of a rolling stock item is known from U.S. Pat. No. 4,656,854 A, where the rolling stock item passes through a rolling stand. Roll eccentricities are identified by the use of a process model and taken into account in the determination of a correction signal for a control device for a final control element of the rolling stand. For the purposes of identifying the roll eccentricities, measured values for the tensile force prevailing in the rolling stock item are fed to the process model.
A similar disclosure can be found in JP 04 200 915 A.